1. Field of the Invention
The invention relates to a power converter, and more particularly, to a switching converter that reduces switching loss without an auxiliary winding implemented.
2. Description of the Prior Art
FIG. 1 illustrates a conventional quasi-resonant fly-back converter, capable of reducing switching loss and switching noise. The converter 100 shown in FIG. 1 includes a transformer T having a primary winding LP and a secondary winding LS, and a power switching transistor Q1 coupled to the primary winding LP of the transformer T. When the power switching transistor Q1 is on, the input energy of the converter 100 originating from the input voltage VIN at the input end is stored into the transformer T, and when the power switching transistor Q1 is off, the energy stored in the transformer T is transferred to the output end, forming a DC output voltage VOUT at the load 101. The switching operation of the power switching transistor Q1 is decided by a regulator circuit comprising a zero-cross detecting circuit 102, a delay circuit 103, a waveform shaping circuit 104 and a control circuit 105 as shown in FIG. 1.
In order to operate the converter 100 in a discontinuous conduction mode (DCM) for reducing the switching loss and switching noise, it is necessary to detect when the primary winding LP of the transformer T is demagnetized; that is, the timing when the secondary current IS reaches zero. In FIG. 1, by measuring a voltage drop of an auxiliary winding LB, the demagnetization timing of the transformer T is detected, and the regulating circuit then turns on the power switching transistor Q1 to reduce the switching loss. Further description associated with the operation of each circuit component shown in FIG. 1 is detailed in U.S. Pat. No. 5,497,311, and is omitted here for brevity.
The auxiliary winding LB, however, represents an additional element that increases manufacturing cost. Elimination of the auxiliary winding LB used to detect demagnetization is beneficial as it reduces the overall manufacturing cost of the converter 100.
FIG. 2 shows another conventional structure of a quasi-resonant fly-back converter 200 without an auxiliary winding. In the quasi-resonant fly-back converter 200, when the secondary current IS reaches zero, the voltage across a drain and a source of the power switching transistor 201 drops off sharply, thereby generating a negative spike in the gate voltage VG. The negative spike is detected by a comparator 202 using a threshold voltage VTH, and the comparator 202 provides a DEMAG signal to a PWM (pulse width modulation) regulator 203 according to a comparison result. The PWM regulator 203 enables the power switching transistor 201 by controlling a first transistor S1 and a second transistor S2 to turn on the power switching transistor 201. Further description associated with operation of each circuit component shown in FIG. 2, such as the one shot circuit 206 and the feedback circuit, etc. is detailed in U.S. Pat. No. 6,469,484, and is therefore omitted here for brevity. Although the fly-back converter 200 does not need an auxiliary winding to detect the demagnetization of the transformer 204, it still utilizes a complicated demagnetization detection circuit. The resistor 205 with high resistance and the one shot circuit 206 both raise the manufacturing cost accordingly.